1. Field of the Invention
The present invention relates to a data transmission system in which a transmission path is shared by a plurality of communication apparatuses and which performs data transmission using synchronous channels, as well as a data transmission method, a data transmission apparatus, and a data reception apparatus, and particularly relates to the system, method and apparatuses which achieve synchronization of operations between the transmission apparatus and reception apparatus connected to each other via the transmission path.
2. Background of the Related Art
In recent years, the study of the communication method for transmitting, via a common bus, digital data such as digitized video, audio data, computer data and the like is eagerly made. A conventional transmission system, for example, the communication system disclosed in Japanese Published Patent Application No. Hei. 9-107373, will be described as an example, using FIG. 14. In FIG. 14, reference numeral 100 denotes a transmission path, and numeral 101 denotes a clock control means for controlling a clock of the transmission path 100. Further, reference numeral 1410 denotes a transmission apparatus comprising a data generation means 1411 and a first communication control means 112, and reference numeral 1420 denotes a reception apparatus comprising a data processing means 1421 and a second communication control means 122.
Hereinafter, the operation will be described.
The transmission system shown in FIG. 14 transmits data from the transmission apparatus 1410 to the reception apparatus 1420 via the transmission path 100. The transmission path 100 operates on the basis of a clock which is controlled by the clock control means 101, and the first communication control means 112 in the transmission apparatus 1410 and the second communication control means 122 in the reception apparatus 1420, which are connected to the transmission path 100, extract the clock when receiving data from the transmission path 100, and utilizes the clock for data processing inside.
The transmission apparatus 1410 inputs the extracted clock to the data generation means 1411. The data generation means 1411 generates transmission data using the input clock and transmits the transmission data to the first communication control means 112. The first communication control means 112 outputs the transmission data to the transmission path 100.
In the reception apparatus 1420, the second communication control means 122 receives the transmission data and transmits the same to the data processing means 1421. Further, the second communication control means 122 simultaneously extracts the clock and transmits the clock to the data processing means 1421. The data processing means 1421 uses the extracted clock to process the transmission data.
In the procedure described above, the data generation in the data generation means 1411 and the data processing in the data processing means 1421 are performed by using the same clock, and thereby synchronization between the transmission apparatus 1410 and the reception apparatus 1420 can be achieved and the transmission system performs operation without failure.
The network as described above is used for the vehicle-mounted multimedia network MOST (Media Oriented Systems Transport) using optical fiber for transmission, and the like, and the network and an apparatus connected thereto operate by synchronizing clocks thereof, thereby reducing buffers and the like to be incorporated into the apparatus to attempt reduction in costs or facilitation of connection of various apparatuses to the network.
Moreover, as for the network (IEEE1394) transmission of IEC60958 (i.e., “IEC60958-1 First edition 1999-12”) format, for example, a demodulation apparatus and a signal processor which receive the IEC60958 format data and demodulate the data in a transmission apparatus is disclosed in Japanese Published Patent Application No. 2000-149461.
As described above, in the conventional data transmission system, and data transmission method, the transmission apparatus 1410, reception apparatus 1420, and transmission path 100 operates on the same clock to perform data generation and processing, thereby maintaining synchronization between transmission and reception.
However, in the conventional construction described above, when the data generation means 1411 on the side of the transmission apparatus 1410 has a specific clock and transmits the transmission data on the basis of the specific clock, or in a system where while a clock source of the data generation means 1411 and a clock source of the transmission path 100 are the same, the transmission apparatus 1410 and the transmission path 100 operate, respectively, on the clocks of different rates in specifications, there is a problem that the synchronization in data processing cannot be achieved.
Further, in a system where, while the data generation means 1411 and the transmission path 100 operate on the clock of the same rate in specifications, asynchronization is exactly caused due to clock sources thereof being different from each other, there is a problem that an overflow or underflow of the transmission data occurs in the transmission apparatus 1410 during the operation of the data transmission system.
The present invention is made to solve the above-described problems, and an object of the present invention is to provide a data transmission system, data transmission apparatus, data reception apparatus, and data transmission method which can maintain synchronization between transmission and reception even when the transmission apparatus, reception apparatus, and transmission path do not operate on the same clock.